This invention relates generally to a method and system for detecting malfunctions in AC motor power systems and, more particularly, to a method and system for detection of incipient failures in an inverter power circuit.
It is common in AC electric motor drive systems to employ power inverter systems to furnish electric power from a DC source to an AC motor. These inverter systems are of various types, but are often comprised of gate turnoff thyristors (GTO) in a bridge arrangement. The GTOs in the bridge are selectively gated to control the electrical power supplied to the motor by converting DC power from the DC source into AC power which drives the motor. Typically, two GTOs are connected in a series arrangement in what is commonly referred to as a "leg" between relatively positive and relatively negative busses of the DC source. A common converter of this type is a three-phase converter having three legs connected in mutual parallel between the positive and negative DC source busses. The GTOs of each of the legs are rendered conductive in a predetermined order or sequence in order to control the electrical power delivered from the DC busses to the AC motor.
The above described motor systems require regular maintenance to avoid or detect a variety of common failure modes. For example, if both GTOs of a leg were to become conductive simultaneously, there would exist a-short between the DC source busses which, if allowed to continue, could result in great damage to the motor, power source, and/or to the GTOs. In addition, failure of various components and segments of the system such as the feedback circuits, drive circuits, and the motor load itself can occur. Generally, expensive and time consuming manual off-line testing has been necessary to detect many errors at an early enough stage to ensure proper operation. U.S. Pat. No. 5,363,039 assigned in common to the same assignee of the present invention discloses self-test techniques for AC motor systems that enhance low-cost maintenance of such systems and that allow for detecting hard failures of the system, such as short circuits, prior to initiating operation of the inverter. In order to further enhance cost-effective maintenance of such systems, it would be desirable to provide techniques that allow for early detection of incipient failures in the system. It would be further desirable to monitor predetermined electrical parameters of the system so as to be able to, for example, detect trends that may be indicative of such incipient failures. It would also be desirable to be able to compensate for deviations from the predicted values of the monitored parameters due to various external parameters, such as ambient temperature, tractive load, traction motor RPM, etc. If uncompensated, such deviations could lead to mistakenly logging faults for the system and in turn this could lead to costly delays and added costs due to unnecessary maintenance.